Question: Which of these numbers is prime? ${17,\ 30,\ 33,\ 45,\ 49}$
A prime number has exactly two factors: 1 and itself. The factors of 17 are 1 and 17. The factors of 30 are 1, 2, 3, 5, 6, 10, 15, and 30. The factors of 33 are 1, 3, 11, and 33. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 49 are 1, 7, and 49. Thus, 17 is a prime number.